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The Last Word

Sour cream

Q: One evening in the summer of 1994 I retired to bed with a glass of milk.
During the night there was a tremendous thunderstorm with plenty of lightning
and the following morning the remainder of the milk had curdled in to a solid
mass.

My elderly relatives who remembered pre-refrigerator days held it as common
knowledge never to leave milk out in a thunderstorm. I had never heard of this.
What process had taken place?

(continued)

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A: I read with interest the replies to the query about milk curdling during
thunderstorms. The explanations offered—that the warm, humid conditions
during storms encourage bacterial growth thereby souring the milk—do not
explain the phenomenon that I observed. Intrigued by the original question I
decided to test it myself and deliberately left a covered glass of fresh,
pasteurised milk taken straight from the fridge on the back doorstep during a
thunderstorm. Within 15 minutes the milk in the glass had separated into a clear
whey-like layer overlaying a layer of curd. Tasting the remixed milk confirmed
that it had not turned sour, only curdled.

The remaining milk in the bottle from which the glass was filled had been
kept in the fridge and remained unaffected.

Why was this?

Val Dawson

Amersham, Buckinghamshire

It’s good to see New Scientist readers are prepared to experiment
for themselves. Can any reader offer an explanation for this new study? –
Ed

Frozen food

Q: My mummy says I must eat fruit and vegetables and not too much fat. Inuit
only eat fat, meat and fish. Why don’t they get heart disease or scurvy?

A: The low heart disease rate among Inuit was observed 30 years ago. To
understand their case, some background information on heart attacks is
necessary.

One of the causes of a heart attack is fatty deposits in the arterial walls
caused by cholesterol and triglycerides in the blood. These deposits block the
artery which slows down the blood flow to the heart. This can eventually cut off
the oxygen supply to the heart. Cholesterol is in all our body cells and we
can’t live without some of it. In the body, some cholesterol is oxidised and
this changes it into a form that allows the fatty deposits to build up. Fresh
fruit and vegetables help stop the oxidisation of cholesterol and so reduce the
number of fatty deposits that develop.

As well as having a low rate of heart attacks, the Inuit are also known for
eating a lot of fish. Studies of fish oil found it contained omega-3
polyunsaturated fatty acids which appeared to lower the levels of cholesterol
and triglycerides in the blood. This reduces the amount of fatty deposits in the
arterial wall. Omega-3 fatty acids may also have a thinning effect on the blood
by reducing the stickiness of blood platelets. This reduces the chance of a
blood clot.

Heart attacks have many causes, including high blood pressure, hereditary and
genetic factors, racial characteristics, dietary factors, lack of exercise and
behavioural factors such as smoking cigarettes. The Inuit seem to avoid most of
these problems. However, the biology of Inuit populations is poorly documented,
so there could be some other factor that helps them avoid heart attacks in
addition to the fish in their diet.

Claire Adams

Perth, Western Australia

A: While the Inuit diet is higher in meat and fat, and lower in vegetables,
than most scientists recommend, it does include berries. Blueberries grow north
of the Arctic Circle, and are readily available in the summer. While we need
vitamin C to avoid scurvy, we don’t need very large quantities of it.

Also, the current Inuit diet is not limited to Arctic foodstuffs. Many people
even in remote parts of the Arctic and including some Inuit, buy food imported
from the temperate zone. This includes fruit and vegetables.

Vicki Rosenzweig

New York, USA

Under and out

Q:: When travelling by car I listen to AM radio broadcasts and it is annoying
that the radio signal almost disappears when I pass under any shape of bridge
and through tunnels. Why doesn’t this happen with FM stations?

A: The question of radio waves penetrating under road bridges is a special
case of the problem of radio propagation down pipes and tunnels. A road bridge
is like a very short tunnel.

We can consider three cases; empty tunnels with electrically conducting
walls, empty tunnels with walls that are not electrically conducting and tunnels
containing electrical conductors such as pipes or cables running along their
length.

In principle, the presence of conductors in a tunnel permits radio waves of
all wavelengths to propagate, sometimes a good distance but often perhaps only a
few tens of metres, depending on the electrical characteristics of the walls and
conductors. Generally, the lower the frequency the greater distance the
propagation.

Tunnels with electrically conducting walls and no separate internal
conductors are like conventional waveguides used in microwave radio
communication. These will carry only those signals whose wavelength is shorter
than twice the dimension of the tunnel cross-section. Microwave signals (perhaps
of a wavelength of 10 centimetres) can sometimes propagate for hundred of metres
down such tunnels, but they are blocked by corners and bends or by obstructions
such as vehicles.

Signals with a wavelength too large to meet the size criterion mentioned
above will only penetrate a fraction of a wavelength down the tunnel. In these
cases medium-waves might go a little farther than VHF, but not very far.

Normal tunnels with, say, concrete walls but without internal conductors also
act as waveguides—but the power of the propagating waves is quickly
absorbed by the walls, so the waves don’t travel far.

In general, the shorter the wavelength of the radio wave the farther it will
penetrate the tunnel before colliding with a wall and being absorbed.

In order to get good radio communication in a tunnel, it is usual to feed VHF
signals into a special type of low-loss coaxial cable running down the tunnel’s
length. The cable is designed to allow a controlled amount of leakage to take
place (like a hosepipe with holes in it) so that radio waves are leaked into the
tunnel at regular distances. This way each part of the tunnel has its own radio
wave supply. This is done in the Dartford Tunnel, where good BBC signals can be
heard. Mines and underground railway systems often use the same technique.

Quintin Davis

Leatherhead, Surrey

This week’s questions

White out: How do mountain-dwelling birds and mammals protect themselves from
snow-blindness?

John Davison

Nelson, New Zealand

Push bike: Why does it seem harder to cycle up a steep hill than to push the
bike up at a similar speed?